There are two kinds of wallet hot wallet and a cold wallet. Ideally, people keep minimum balance for trading in a hot wallet and most of the coins are stored in cold wallet. I am wondering why coins in the cold wallet are not tradable?

3 Answers
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The very point of a cold wallet is that it's less easily-accessible to provide additional security guarantees. By storing and interacting with your private keys offline, there is a smaller attack surface for bad actors trying to gain access to your private keys. By removing that friction, you're making that cold wallet a hot wallet by definition.

Actually, coins on a cold wallet machine CAN be traded ( I'm talking about a dedicated computer here). A transfer can be generated on a connected machine, giving a transaction ID, basically a large number. This can be transferred by some medium to the airgapped machine with the cold wallet (a QR code is the most secure way). You then sign the transaction with your private key and generate a new, signed code. This signed code is transferred back to the connected machine which then broadcasts the transaction to the blockchain. It's secure because your private key never leaves the cold wallet, which is airgapped.